This invention relates to the synthesis of audio signals from stored digital data and more particularly to a synthesizer in which the stored data are sets of the coefficients of a recursive filter and where each set is applied to the filter for a fixed time period, the periods totaling the duration of the synthesized audio signal.
Acoustic trainers are typically required to produce signatures characteristic of signals received from sources in a real ocean environment. Traditionally, the broadband and harmonic spectral content of targets and the broadband content of background noise have been emphasized for replication. Recently, active echoes and reverberation have been added to the trainer repertoire. An additional component of the acoustic environment which is required for purposeful training is the set of transient signatures. These include occasional and also continuous biologic emissions, hatch openings and closings, ice fractures in the arctic environment, undersea seismic disturbances, and the noise of submerged wrecks moving with currents--just to name a few. The synthesis of these transients has typically resided in an instructor controlled analog tape recorder. The disadvantage of this approach is the large number of tapes required and/or the problem of and time required for locating a particular sound of a number of sounds on a long tape. In addition, the control of the tape recorder and its connection are cumbersome.
It is therefore an object of this invention to provide a computer controlled synthesis system providing transient audio signals. It is also an object of this invention to provide a system which is not cumbersome and is easy to use in the selection of different stored transient sounds. It is a further object of this invention to provide a digital synthesizer with denser packaging (smaller volume) for storing a large repertoire of audio sound signals. It is a still further object to produce a synthesizer which is more reliable than the analog synthesizer of the prior art.
It is a feature of this invention that the method of synthesis utilizes linear prediction coding techniques to derive time-varying-filter coefficients. These coefficients are stored in digital form and are used to program a recursive filter which is driven by white noise. The resulting signatures are then an inherent part of the trainer and are generated under complete computer control.